HIV-1 infection begins with fusion of viral and target cell membranes. Viral attachment and membrane fusion are mediated by viral envelope spikes upon engagement with host cellular receptors. The mature envelope spikes contain three copies each of noncovalently-associated receptor-binding subunit gp120 and fusion subunit gp41. Despite considerable progress in our understanding of the structure of HIV-1 envelope glycoprotein over the last 15 years, we still do not have an atomic picture of the full-length and fully glycosylated gp120 due to enormous technical challenges associated with crystallographic studies of this protein, which is heavily coated with carbohydrates. Such a structure is, however, critical for a full understanding of gp120 function, as well as its interactions with various ligans, in particular, broadly neutralizing antibodies (bNAbs). Determination of an atomic structure of an intact HIV-1 gp120 will mark an important milestone in structural biology of HIV-1 entry, and may also guide development of antiviral therapeutics and vaccines. We have obtained diffracting crystals of a full-length and fully glycosylated HIV-1 gp120. In this application, we propose to determine crystal structures of the intact and glycosylated HIV-1 gp120 in complex with several neutralizing antibodies. We hypothesize that high-solution crystal structures of the unaltered HIV-1 gp120 will provide novel mechanistic insights into gp120 function, antibody neutralization and immune evasion. We will pursue the following specific aims: 1) We will determine the crystal structure of an intact, fully-glycosylated HIV-1 gp120; 2) We will determine the crystal structure of intact gp120 in complex with 2 domain CD4 and an anti-CD4 antibody ibalizumab; 3) We will determine crystal structures of HIV-1 gp120s with distinct characteristics.